The present invention generally relates to magnetic discs and methods of producing magnetic discs, and more particularly to a magnetic disc comprising a magnetic layer in which an axis of easy magnetization (hereinafter simply referred to as magnetic easy axis) is oriented in a circumferential direction of the magnetic disc and a method of producing such a magnetic disc.
For example, a conventional magnetic disc comprising a thin film made of a magnetic material is produced by carrying out a magnetron sputtering by use of a ring-shaped magnet. In the present specification, such a magnetron sputtering using the ring-shaped magnet will be referred to as a ring magnetron sputtering. When a non-magnetic underlayer having a film thickness of 3000.ANG. to 4000.ANG. by use of chromium (Cr) and a magnetic layer having a film thickness of 700.ANG. to 800.ANG. by use of cobalt-nickel (CoNi) are formed on the magnetic disc by the ring magnetron sputtering, it is only possible to obtain a coercive force (coercivity) Hc of 795 Oe, a squareness ratio Rs of 0.71, a coercive squareness S* of 0.74, a resolution of 91.9%, a half-value width W.sub.50 of the solitary reproduced waveform of 1.96 microns, and a density characteristic D.sub.50 of 16.82 kilobits per inch (KBPI). Accordingly, the static magnetic characteristic and the magnetoelectric conversion characteristic of the magnetic disc produced by the conventional method are unsatisfactory.
The present inventors studied in detail the magnetic disc produced by the conventional method, and found that the reason why the static magnetic characteristic and the magnetoelectric conversion characteristic of the magnetic disc produced by the conventional method are unsatisfactory is because the magnetic easy axis of the magnetic layer is oriented at random. In addition, the present inventors also found that the static magnetic characteristic and the magnetoelectric conversion characteristic of the magnetic disc are greatly improved when the magnetic easy axis of the magnetic layer is oriented in the circumferential direction of the magnetic disc.
However, according to the physical vapor deposition (PVD) such as the conventional sputtering, it is impossible to form the magnetic layer of the magnetic disc so that the magnetic easy axis of the magnetic layer becomes oriented in the circumferential direction of the magnetic disc.
On the other hand, a method has been proposed according to which a spiral groove or a plurality of concentric grooves are formed on a substrate and the magnetic layer is thereafter formed on the substrate by the conventional sputtering. In this case, since the groove or grooves on the substrate are formed in the circumferential direction of the magnetic disc, the magnetic easy axis of the magnetic layer becomes oriented in the circumferential direction due to the shape anisotropy thereof although the magnetic layer is formed by the conventional sputtering. However, according to this proposed method, there is a problem in that the substrate must be heated during the sputtering in order to ensure that grains of the magnetic layer grow in the circumferential direction of the magnetic disc. In other words, the substrate must be heated so that the grains of the magnetic layer become deposited on the substrate as chains having long axes of the grains aligned substantially in the circumferential direction. As a result, this proposed method suffers problems in that there is a need to form the groove or grooves on the substrate and that it is impossible to use a material such as plastic which cannot withstand heat for the substrate which must be heated during the sputtering described above.